A variety of implantable medical devices have been developed to support various body functions. Examples of these devices include implantable pacemakers and implantable cardioverter/defibrillators (ICDs) for monitoring and for stimulating ailing hearts. Implantable bladder stimulators provide electrical stimuli to bladder muscles to restore bladder function. Similar types of electrical stimuli provided by bone growth stimulators help the patients with complicated bone fractures. Cerebellar implantable devices monitor brain activities and stimulate the brain to control seizures in epilepsy as well as pain.
Often implantable medical devices are provided with one or more leads for implant with particular organs for sensing signals therein and for delivering therapeutic stimulation. In the case of a pacemaker or ICD, the leads have electrodes for mounting within the chambers of the heart. The electrodes allow electrical cardiac signals to be sensed within the heart and further allow pacing pulses or other therapeutic stimulation pulses and shocks to be delivered directly to the heart. State-of-the-art pacemakers and ICDs are typically equipped with two or three leads, each having two or more small electrodes for pacing/sensing as well as at least one larger coil electrode for delivering powerful cardioversion or defibrillation shocks. Insofar as pacing/sensing is concerned, some leads are “unipolar,” i.e. the lead includes only a single pacing/sensing electrode, referred to as a tip electrode. Electrical cardiac signals are sensed between the tip electrode and the housing of the device. Pacing pulses are also delivered between the tip electrode and the device housing. A single conductor is provided within the lead to conduct electrical signals to/from the tip electrode. Other leads are “bipolar,” i.e. the lead includes a pair of pacing/sensing electrodes, referred to as tip and ring electrodes. Electrical cardiac signals are sensed between the tip and ring electrodes of the lead. Pacing pulses are also delivered between the tip and ring electrodes of the lead. Two isolated conductors are provided within the lead to separately conduct electrical signals to/from the tip and ring electrodes. These conductors are in addition to any conductors provided for connection to the separate coil electrodes. Further information regarding lead/sensor designs may be found in: U.S. Pat. No. 5,275,171 to Barcel, entitled “Implantable Lead and Sensor”; U.S. Pat. No. 5,431,681 to Helland, entitled “Combination Pacing and Defibrillating Lead Having Sensing Capability”; U.S. Pat. No. 5,438,987 to Thacker, et al., entitled “Implantable Lead for Sensing a Physiologic Parameter of the Body”; and U.S. Pat. No. 6,591,143 to Ekwall, entitled “Bending Sensor for an Implantable Lead and a Heart Stimulator with a Lead having such a Sensor.”
In many cases, it is also desirable to equip the lead with one or more physiological sensors for sensing various other signals or parameters of interest. In the case of a pacemaker or ICD, physiological sensors may be provided, e.g., for sensing the oxygen content of blood, the pH of blood, the temperature, blood glucose levels, pressure, etc. Within conventional leads, each sensor requires one or more dedicated conductors for feeding power and control signals to the sensor and for receiving physiological output signals from the sensor (such as signals representative of blood glucose levels, pH levels, etc.) The addition of these conductors within the lead further requires additional connectors/filters within the device itself, such as electromagnetic interference (EMI) filters, and also reduces lead flexibility. Hence, even the addition of a single sensor to the lead significantly increases the complexity of both the lead and the device, thus elevating design and fabrication costs while potentially reducing reliability. The addition of two or more sensors on a single lead still further inflates cost and complexity. As a result, pacemaker and ICD manufacturers often do not provide sensors on leads. Hence, the patient does not benefit from the many advantages that such sensors would provide. As just one example, the provision of a blood glucose sensor on the lead would allow the pacemaker or ICD to easily detect hypoglycemia, hyperglycemia, or other blood glucose level-based conditions, provide suitable warning signals, and control therapy in response thereto.
Accordingly, it would be desirable to provide techniques for accommodating one or more sensors within implantable leads without requiring additional conductors along the leads and the corresponding additional connectors/filters needed within the device itself. It is to this end that the invention is generally directed.
Some attempts have been made to reduce the number of conductors associated with physiological sensors within leads. See, for example, U.S. Pat. No. 6,163,723 to Roberts et al., entitled “Circuit and Method for Implantable Dual Sensor Medical Electrical Lead,” which describes a unipolar lead arrangement wherein only a single additional conductor is required within the lead to accommodate a pair of sensors. Briefly, a pair of physiological sensors is coupled between a standard tip lead conductor and an additional return path conductor. As best as can be understood from the descriptions, pacing pulses are delivered via the tip electrode only while the return path conductor within the lead is electrically disconnected. As such, the only return path from the tip electrode to the device is through patient tissue, allowing the therapeutic pacing pulses to be delivered to the tissue. Sensing of electrical cardiac signals via the tip electrode is also performed while the return path conductor within the lead is disconnected. To sense physiological parameters using the physiological sensor, the return path conductor of the lead is instead electrically connected, such that a complete circuit is provided through the sensors. Electrical current is then routed through the pair of sensors and directly back to the device via the return path electrode. The polarity of the current is switched to alternatingly activate just one of the sensors. In other words, only one sensor can be activate at any given time. Moreover, when the sensors are active, therapy cannot be delivered via the tip electrode, and vice versa.
As can be appreciated, there are certain potential disadvantages with this approach. The two sensors cannot be used simultaneously. In some cases, it may be desirable to sense two separate physiological parameters concurrently. Moreover, physiological sensing cannot be performed while pacing pulses are concurrently being delivered. Again, circumstances may arise where it is desirable to sense physiological parameters at the same time that a pacing pulse is delivered. Still further, the approach seems to require an even number of sensors (i.e. two per each additional return path conductor). In many cases, it is desirable to provide only a single physiological sensor, or some other odd number of sensors. Perhaps even more significantly, the technique requires additional return path conductors (one per each pair of physiological sensors). It would be far preferable to accommodate physiological sensors without requiring any additional conductors whatsoever within the lead.
Note also that some techniques have been developed that exploit bus-type arrangements to accommodate multiple sensors within a single lead. See, for example, U.S. Pat. No. 5,593,430 to Renger, entitled “Bus System for Interconnecting an Implantable Medical Device with a Plurality of Sensors” and U.S. Pat. No. 5,999,848 to Gord et al, entitled “Daisy Chain Sensors and Stimulators for Implantation in Living Tissue”. These patents describe lead arrangements wherein a pair of additional conductors is provided within a lead for accommodating a set of sensors. Bus-type control schemes are employed to separately and individually activate and control the various sensors. Although only two additional conductors are required within the lead to accommodate an arbitrarily large number of sensors, these conductors are in addition to the tip and ring conductors, which are used for pacing/sensing via the tip/ring electrodes. Again, it would be far preferable to accommodate physiological sensors without requiring any additional conductors within the lead.
One design that succeeded in eliminating the need to provide an additional conductor is described in U.S. Pat. No. 5,411,532 to Mortazavi, entitled “Cardiac Pacemaker having Integrated Pacing Lead and Oxygen Sensor.” Briefly, an oxygen sensor is coupled along a conductor leading to a pacing electrode. That is, the sensor is electrically connected in series with the pacing electrode along a single conduction path leading from the device through the sensor to the stimulation electrode and then back to the device via tissues of the body. The oxygen sensor is configured to receive and respond to current pulses having a polarity opposite that of pacing pulses applied to the heart muscle. A diode distinguishes between pacing pulses and oxygen sensing pulses according to the direction of current flow. Although the series design of Mortazavi advantageously allows a physiological sensor to be provided within the lead without requiring additional conductors, it does not permit the physiological sensor to sense signals during delivery of a pacing pulse. Also, it does not appear that any additional physiological sensors can be readily accommodated. Also pacing pulse amplitudes would be decreased by a diode drop, thus reducing pacing efficiency. It would be preferable to accommodate multiple physiological sensors without requiring any additional conductors within the lead.